Pre-orders for Apple's new iPhones are off and running, and there's already a clear winner: the 5.5-inch iPhone 6 Plus is sold out, well ahead of the "standard" model, the 4.7-inch iPhone 6.

The big screen is officially the big hit.

It's certainly possible Apple made more iPhone 6's than it did 6 Pluses, but early buzz shows interest in the Plus model is on par with its little brother. A poll on Polar shows excitement for the two phones is about even, and when Today's iPhoneasked readers which model they bought, the Plus got almost half the votes. Even if Apple made fewer Pluses available at launch, as some suspect, the sheer speed of the selloff is still impressive.

The market for large-screen phones — often called phablets since their size class is between phones and tablets — has been growing for a while. In Asia especially, smartphone buyers are opting for big displays, ostensibly to negate the need for a standalone tablet. On the Android side, Apple rivals Samsung, HTC and LG have been gobbling up market share with their phablet offerings.

It didn't take a fortune teller, let alone those leaked photos, to predict that Apple would respond with a big-screen iPhone. On the eve of the iPhone launch, Adobe published a fairly definitive study that showed web browsing on phones larger than 4 inches was increasing while usage on smaller phones was decreasing.

Demand for phablets is clearly shooting up, but what's behind those market forces? Why do people like phablets so much? More to the point, why are people opting for phablets in spite of their usability trade-offs? Sure, everyone wants a bigger screen, but if size, weight and one-handed operation weren't concerns, we'd all be making phone calls on iPads, which some people are actually starting to do.

This trend toward ever-larger screens can't be simply dismissed as an irrational desire for "moar" of something tantalizing that's actually bad for you, like eating too many cupcakes, as mobile analyst Sascha Segan has argued. It's a reflection of two conjoined trends: Mobile app experiences are getting more sophisticated, and phone design has progressed to a point where the trade-off in usability isn't such a bad deal after all.

The rise of gestures

Mobile operating systems today are pretty damn mature. A couple of years ago apps were just starting to talk to each other and multitasking was relatively new. Today, we expect almost the same functionality from a mobile app that we do from a desktop one, and those apps, by and large, are delivering, providing most if not all of their functionality on smartphones and tablets.

Something's happened in mobile user-interface design in the last couple of years to help with this: gestures. In the context of smartphones, I'm referring to gestures as swiping in from an edge or doing something with your fingers besides the usual tap. Android has had them for a while, and BlackBerry based its entire BB10 platform around them. iOS 7 brought them full-force onto the iPhone.

These edge swipes let you hide navigation and menus away so a designer doesn't need to waste too much screen real estate on virtual buttons, but gestures have a side benefit to the user: You don't need to stretch your thumb as much.

The Google+ app is a great example: A user can call up the nav from the left side of the screen by swiping anywhere along the edge — it makes no difference whether you do it on the top or bottom. Facebook works similarly: Swipe from the right side, anywhere, to message any of your friends.

Gestures have helped make phablets more usable. But they're not the whole story; predictive algorithms and learning keyboards such as Swype mean we're less reliant on actually typing every single character when tweeting, texting or replying to emails. That has benefits for all screen sizes, but for phablets, it makes one-handed operation less awkward.

At the same time, better multitasking means switching from app to app doesn't involve hunting through big arrays of app icons. Context-based systems such as Google Now let us browse data from multiple apps on simple gesture-controlled cards. Throw in voice assistants like Siri and the questionable but unquestionably efficient language of emoji, and it's a wonder we still need to type at all.

Smartphone makers recognize, though, that using a phablet with one hand can often be a problem (NBA players excepted), and many offer tools to address that specific pain point. Samsung has included a one-handed mode with every Galaxy Note phone, which slightly shifts the keyboard and some navigation items to one side or the other. For Apple's part, it offers "reachability," which shifts the entire screen downward with a double-tap of the home button.

Momentous design

Hardware matters, too. Smartphone design has improved greatly in the last couple of years, with direct benefits to the usability of phablets. If you looked at last year's Moto X, you would not believe the phone has a 4.7-inch screen, but it does. The footprint of the phone is on par with an iPhone 5, yet it remains small and light, thanks to a slim bezel and custom parts like its curved battery.

The Moto X isn't just a one-off, either. Sharp recently showed a phone, the Aquos Crystal, with an "edge to edge" display that makes the bezel almost disappear. The Google Nexus 5, with a 5-inch display, is so light you might think it doesn't even have a battery until you turn it on. And the iPhone 6 Plus might be the largest iPhone ever, but it's also one of the thinnest at 0.28 inch. That's even thinner than its predecessor, the iPhone 5S.

The Sharp Aquos Crystal

Even with all these benefits to phablet usability, many people will still prefer a smaller phone. Reachability is one thing, but it does nothing for pocketability, which matters to many. For people with smaller hands or who often use their phones with one hand (think subway commuters), the UI challenges of a huge screen are exacerbated. And then there's just the totally normal desire for compactness — to have a gadget that does everything you want in as small and light a device as possible.

But the phablet is here to stay, and it's not too hard to see why. Today you can get a a big, beautiful screen and make far fewer compromises than you did a couple of years ago. The category is certainly a big part of the reason tablet sales have stagnated; after all, why use a tablet when your phone is almost as big, and it's always close by?

The world laughed at Samsung when it introduced the first phablet back in 2011, but the craze for the big-screen iPhone is its final vindication. The future of smartphones is — literally — enormous.

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